(1) Field of the Invention
The present invention relates to a monitoring device for monitoring a power transmission system of an aircraft, to an aircraft fitted with the monitoring device, and to the method used.
(2) Description of Related Art
In particular, the aircraft is a rotorcraft having a main rotor providing the rotorcraft with at least part of its lift and possibly also propulsion. The rotorcraft also has at least one auxiliary member for controlling its movement in yaw.
In particular, and by way of example, a helicopter may include both a main rotor and an auxiliary member that are driven in rotation by a power plant. Such an auxiliary member may for example be provided with a rotor that is referred to below as an “auxiliary” rotor.
Furthermore, the power plant is mechanically connected to each rotor by a power transmission system.
Helicopters are provided with a power plant that comprises at least one engine. By way of example, such an engine may be a free-turbine turboshaft engine. Each engine has a drive shaft that rotates at high speed. In contrast, the main rotor of a helicopter rotates at a low speed, lying substantially in the range 200 revolutions per minute (rpm) to 400 rpm. Under such circumstances, the power transmission system includes a gearbox for reducing speed of rotation, which gearbox is interposed between the engines and the main rotor. Such a gearbox for reducing speed of rotation is referred to as a main gearbox (MGB).
Consequently, each engine is connected to the main gearbox of a power transmission system, the main gearbox being connected to the main rotor and possibly also the auxiliary rotor.
In particular, the main gearbox drives a rotor mast in rotation. The rotor mast then drives the main rotor in rotation.
Furthermore, the main gearbox drives the auxiliary rotor in rotation via an auxiliary power transmission and a tail gearbox. Under such circumstances, the tail gearbox may be interposed by way of example between the auxiliary power transmission and the auxiliary rotor.
The person skilled in the art uses the term “rear tail unit” to designate the unit comprising the auxiliary power transmission, the tail gearbox, and the auxiliary rotor. This unit is also referred to below as the “auxiliary” unit.
Furthermore, in order to guarantee mechanical integrity of the power plant, three engine-monitoring parameters are conventionally defined and limited, namely: the temperature known as “T4” of the gas in the combustion chamber of each engine; the speed of rotation Ng of the gas generator in each engine; and the torque Tq exerted on a drive shaft of each engine that is connected to the main gearbox.
In addition, a manufacturer applies limits in order to protect the inlet of the main gearbox, the rotor mast, and the auxiliary unit from excessive torque.
A manufacturer then tends to protect a power transmission system by setting limits for torque exerted on the rotor mast, torque exerted on the mechanical connections linking the engines to the main gearbox, and torque exerted on the auxiliary unit.
The various limits on the power plant and the power transmission system are set so as to prevent mechanical degradation of the various members concerned.
Which limit is the most constricting depends on the stage of flight. In general manner, at low altitude (i.e. below about 3000 meters (m)) and in the absence of an engine failure, the limit for the torque that may be exerted on the rotor mast is found to be the limit that is the most constraining.
In addition, for reasons of cost and of difficulty in implementing a torque meter, it is only the engines that are generally provided with such torque meters. Each engine thus has a respective torque meter arranged on its drive shaft so as to enable the measured torque to be compared with an intrinsic torque limit of the engine. A torque meter is thus used firstly for the purpose of avoiding the engine exceeding its own torque limit.
Nevertheless, a manufacturer may also use the torque meter of each engine to monitor the power transmission system.
The torque limit for application to each connection linking an engine to the main gearbox can thus be monitored by using the measurements from the torque meters.
Specifically, the power developed by a rotary member or exerted on that rotary member is equal to the torque exerted on the rotary member multiplied by its speed of rotation.
Consequently, and in accordance with the prior art, a manufacturer determines the limit power that can be developed upstream from the main gearbox and deduces a torque limit therefrom. This torque limit is referred to as the “gearbox inlet torque limit” in order to distinguish it from the intrinsic torque limit of each engine.
In order to determine this limit power, the following power relationship is used:Peng=P1+P2
where “Peng” represents the maximum authorized power for the engines, “P1” represents the maximum authorized power on the rotor mast, and “P2” represents the power absorbed by the auxiliary unit.
The power absorbed by the auxiliary unit is generally not associated with a torque limit on that auxiliary unit because of the very high speeds of rotation of the parts in the auxiliary unit.
The maximum authorized power for the engines is defined by mechanical constraints set by the manufacturer of the aircraft, with these constraints generally differing from the limits imposed by the engine manufacturer.
The limit power applicable to each engine is obtained from the maximum authorized power for the engines in application of a predefined distribution of power. For example, distributing power equally between the engines of a twin-engined rotorcraft leads to defining a limit power for each engine that is equal to half the maximum authorized power for the engines.
This power relationship may include a variable that is adjustable, e.g. in order to take account of power losses caused by the operation of the main gearbox. Likewise, this relationship may include a variable that is adjustable in order to take account of the possibility of power being taken off from the power transmission system in order to perform accessory operations.
In addition, the maximum authorized power P1 on the main rotor is a power constant defined by the manufacturer.
In addition, in the absence of any torque measurement on the auxiliary unit in the prior art, the manufacturer evaluates the power P2 absorbed by the auxiliary unit as a function of the stage of flight.
Thus, at low speed, the power P2 absorbed by the auxiliary unit is equal to a first constant. This first constant usually corresponds to an estimate of the power P2 absorbed by the auxiliary unit during a stage of hovering flight, with this stage of hovering flight being performed while the engines are delivering a level of power known as “maximum takeoff power” (TOP) and under the French expression “puissance maximal au &collage” corresponding to the acronym PMD.
During a stage of cruising flight, the power P2 absorbed by the auxiliary unit is equal to a second constant. This second constant usually corresponds to an estimate of the power P2 absorbed by the auxiliary unit during a stabilized stage of level flight, this stabilized stage of level flight being performed while the engines are delivering a level of power known as “maximum continuous power” (MCP).
Consequently, the aircraft has equipment that determines the current stage of flight and that deduces therefrom the power being absorbed by the auxiliary unit. This power P2 absorbed by the auxiliary unit can be referred to as an “assumed” power, given that it is based on assumptions that are not necessarily true under all circumstances.
By summing the maximum power P2 consumed by the auxiliary unit and the maximum authorized power P1 on the rotor mast, equipment on board the aircraft can determine the maximum power Peng that the power plant can deliver while not damaging the power transmission system. Under such circumstances, a gearbox inlet torque limit is established for each engine as a function of the speed of rotation of the members being monitored, and in particular a drive shaft of each engine.
A monitoring device can then display this gearbox inlet torque limit together with the current torque measurement as delivered by the torque meter of an engine.
That method thus makes it possible to monitor a power transmission system while using only the torque meters of the engines. That method presents the advantage of not requiring the use of a torque meter that is located on the auxiliary unit or on the rotor mast.
Nevertheless, for the mechanical limits of the power transmission system, the gearbox inlet torque limit is thus established on the basis of an “assumed” level of power being consumed by the auxiliary unit.
This assumed consumption can turn out to be rather inaccurate, for example when performing a sideways movement during hovering flight or in the presence of a gust of cross wind.
This inaccurate estimate of the power being absorbed by the tail unit can then lead to artificial limits being put on the performance on the aircraft in order to guarantee compliance with mechanical limits under said particular stages of flight.
For example, in order to combat a cross wind while hovering, and as a function of the direction of the cross wind, the cross wind may require thrust from the auxiliary rotor to be increased relative to an equilibrium situation without wind, as was used to quantify the assumed level of power absorbed by the auxiliary unit.
Consequently, the auxiliary rotor consumes power that is greater than the assumed power that has been used to establish the maximum authorized power for the engines on the basis of the power relationship. Under such circumstances, the maximum authorized power for the engines is not sufficient to obtain the maximum authorized power in the rotor mast as used in that power relationship.
As a result, the real power that is actually transmitted to the rotor mast is less than the imposed maximum power. This situation leads to an arbitrarily restrictive limit on the performance of the aircraft under difficult flight circumstances.
The technological background includes the following documents: FR 2 278 576; FR 2 541 725; and U.S. Pat. No. 5,775,090.
Document FR 2 278 576 describes a system for controlling a helicopter having two lift rotors.
Document FR 2 541 725 describes an installation for controlling the distribution of load and the speed of rotation of gas turbine installations.
Document U.S. Pat. No. 5,775,090 describes a method of determining a torque signal for a gas turbine.
Documents EP 1 310 646 and EP 2 749 496 are also known.